The present invention relates to digital measuring instruments and more particularly to a digital measuring instrument which is mounted on the feed shaft of machine tools with which it is used.
The digital measuring instruments displaying the number of revolutions of the feed shaft of the machine tools is known to the art by U.S. Pat. No. 4,117,320 and British Pat. No. 1,499,504. The feed system of the machine tools in general is constructed in such a way that threads are formed on a feed shaft and a nut is screwed into the threads, and the nut is coupled to a transfer unit. Another general construction of the feed system of the machine tools is an arrangement such that a gear is fixed to a feed shaft, and the gear is meshed with a rack, and the rack is coupled to a transfer unit. In the foregoing construction, when the feed shaft is rotated, the transfer unit is transferred in a linear direction. Accordingly, when the digital measuring instrument is mounted on the feed shaft, the rotary motion of the feed shaft is converted to pulse signals and the pulse signals are counted by a reversible counter, and the transfer distance or feed ratio of the transfer unit is then digitally displayed. However, as is well known, a backlash phenomenon exists; namely, there is a play (clearance) which exists between the thread and the nut or between the gear and the rack which form the feed system of the machine tool. For this reason, when the direction of rotation of the feed shaft is reversed, even if the feed shaft is rotated, a phenomenon occurs wherein the transfer unit is not transferred in the linear direction due to the play in the feed system. Accordingly, in the case where the conventional digital measuring instrument is mounted on the feed shaft of the machine tool, the non-load rotary motion of the feed shaft due to the play in the feed system of the machine tool is digitally counted. Consequently, the conventional digital measuring instrument brings about an inconvenience in that the accurate transfer distance of the transfer unit is not displayed. The non-load rotation of the feed shaft due to the play in the feed system is generated when the direction of rotation of the feed shaft is reversed. For this reason, in general, the feed shaft is caused to turn in a direction which eliminates any influence due to the play in the feed system, and then, the counter of the digital measuring instrument is reset to zero so as to effect the standard setting, and thereafter, the feed operation is started in only one direction. In the case wherein the transfer unit is fed in the opposite direction, the influence of the play in the feed system is again removed by the foregoing manner; namely, the counter is reset to zero and the feed operation then started in the opposite direction. As will be obvious from the foregoing description, the operation of eliminating the influence of the play in the feed system whenever the direction of rotation of the feed shaft is reversed is an extremely cumbersome work, giving a rise to an extreme deterioration of the operation efficiency.